If they can agree who goes first, Paris and Los Angeles will be awarded the 2024 and 2028 Olympics.
International Olympic Committee members voted unanimously to seek a consensus three-way deal between the two bid cities and the committee's executive board. Talks will open with Paris widely seen as the favorite for 2024. If a deal falls through, only the 2024 hosting rights will be voted on when the IOC meets on September 13 in Lima, Peru.
However, an agreement seemed assured by the reaction of the two mayors. Eric Garcetti of Los Angeles and Anne Hidalgo of Paris emerged on stage holding hands to welcome the decision.
A deal is also likely because a head-to-head fight to host the 2024 games would create a loser that is unlikely to return four years later for a new 2028 bid contest.
"Both of us will find it more and more difficult to convince cities — whether it's Paris, Los Angeles, or other American cities — to really go into this process if one of us gets turned down," Garcetti said in a news conference on Tuesday.
The mayors were united on stage by IOC President Thomas Bach, who raised an arm of each in a shared gesture of triumph. A deal to make both cities winners would fulfill a strategy that Bach set in motion last December to help safeguard a stable future for the signature Olympic event.
"With Los Angeles and Paris, there are two fantastic cities from countries with a profound Olympic history," Bach said.
The IOC approved the expected double award after hearing the cities presented their 2024 hosting plans at a conference center in the Olympics' capital city, Lausanne. Both cities used 45 minutes of videos and speeches, including one with French President Emmanuel Macron promoting the Paris cause, in a closed-door session with IOC members to explain how they would host the 2024 Olympics.
At separate news conferences, the mayors said they could work toward a deal.
"We look forward to working together, maybe not in competition but collaboration with Paris," Garcetti said after his city's bid officials opened the campaign event.
Both mayors have long touted their good relations on other issues, such as climate change.
"We are all at the disposition and by the side of the IOC which was right to ask itself this question," Hidalgo said at the Paris news conference, citing her friendship with Garcetti as potentially a "key element."
The dual award can give the IOC a decade of stability with two world-class cities touting financially secure bids. L.A. plans to use only existing venues with zero risk of white elephants. This follows years of overspending by Olympic hosts and a series of political defeats that have sunk the campaigns of potential candidates.
It also avoids inflicting a third recent defeat on Paris — which lost with bids for the 2008 and 2012 Olympics — and the United States. New York and Chicago both lost heavily for 2012 and 2016, respectively.
Those losses deepened a rift between the U.S. Olympic Committee and the Swiss-based IOC that L.A. 2024 and a new team of American officials have worked hard to heal.
Paris also failed with a 1992 bid and pinned its hopes on hosting in 2024, exactly 100 years after its previous Summer Games.
"We lost three times, we don't want to lose a fourth one," Macron said at the news conference. "I'm here to convey the message that there's a strong unity to back this candidacy."
Minutes after Macron spoke, U.S. President Donald Trump wrote on Twitter: "Working hard to get the Olympics for the United States (L.A.). Stay tuned!"
Garcetti said the Olympic movement "can't afford to lose the United States."
The IOC's most valuable TV rights deal is with NBC and several of its top-tier sponsors are American. Still, a 2028 Olympics in Southern California could be the first American-hosted games since 1996 in Atlanta.